The aim of the proposed project is to construct and use a laser scanning device capable of recording rapid and highly detailed maps of electrical impulse conduction in normal and arrhythmic hearts. To this end the instrument will be capable of monitoring simultaneously electrical activation (action potential upstrokes) from 500-1000 sites with a spacial resolution of 50 micrometers from epicardial surface of the heart. The technique takes advantage of recent developments where membrane action potential can be measured remotely by staining a heart with voltage-sensitive fluorescent dyes. The voltage sensitive fluorescent dye WW781 was found to have large signal to noise ratio with little or no photobleaching or phototoxicity in the heart. The electrical activity from as much as 1000 discrete sites will be measured almost simultaneously (within 7 msec) by scanning the surface of the heart with a "roving" laser beam (wavelength of 632.8nm). The laster positioning is achieved by directing the beam through two acousto-optical devices controlled by a minicomputer. The laser beam can be thus positioned at any random site on the heart. The flexibility of the laser scanner makes it possible not only to scan large surfaces of heart but also to examine conduction anomalies in small patches of heart, SA nodal cells, and purkinje fiber networks. We have already demonstrated the capabilities of the cardiac laser scanner (Dillon & Morad, Science, 214, 1981) and will proceed to improve and modify the technique such that it can be used in dogs in order to examine the normal and arrhythmogenic patterns of conduction. Since disruption of normal cardiac rate and rhythm is one of the major causes of heart disease, a number od iagnostic techniques have been developed in order to assign proper pharmacologic and surgical therapy to the variety of tachycardias, fibrillation and asystoles that are encountered clinically. These methods have also been employed as research tools to uncover the mechanisms of arrhythmias and their vulnerability to certain drugs and procedures. The cardiac laser scanner technique can bring an unprecedented degree of resolution and flexibility to these efforts and may in fact resolve some controversies and solve some of the riddles of arrhythmogenesis.